This invention relates to building construction and reinforcement in general, and to an improved continuity tie and continuity system for strengthening the performance of building structures in response to forces generated by seismic waves and other external forces.
A continuity tie is part of a continuity system which is an integral part of a building's structural roof or floor system specifically designed to resist lateral forces from events such as earthquakes, wind, or blast. A continuity system generally consists of a plurality of spaced continuity lines that extend completely across both the length and width of a building. The purpose of a continuity system is to provide for a discrete structural mechanism for the transference of loads through a roof or floor diaphragm, such as for the transference of lateral forces that might be generated by a concrete wall panel during an earthquake to the structural elements intended to resist such forces.
In buildings with timber framed roofs and floors, and plywood diaphragms, continuity lines often incorporate otherwise required structural members such as purlins or glulam beams. In the absence of an element that is specifically designed to connect these members to form a continuity line, the forces normally transferred through the continuity line are transferred through the plywood diaphragm. At those locations where the plywood is discontinuous across a structural member the continuity forces are transferred via a mechanism that incorporates the nailing between the plywood and the structural member. Such a load transfer mechanism subjects the plywood nailing to out-of-plane loading and the structural member to cross grain tension. The nailing along the edges of a sheet of plywood is generally intended to resist only in-plane loading (parallel to the plywood sheet edges), and is prone to damage or failure when subjected to out-of-plane loading. Typically the cross grain tensile capacity of timber is poor and load transfer mechanisms that subject a timber element to cross grain tension are prone to failure. In areas subject to high seismicity or wind loads, the deficiencies associated with continuity systems that only incorporate the roof and/or floor diaphragms are understood, and buildings are generally designed with continuity systems that incorporate the structural roof and/or floor framing members into discrete continuity lines with continuity ties interconnecting these members. A common specifically designed continuity tie is some type of bolted assembly.
The bolted assembly commonly used as a continuity tie is referred to as a holdown. An example of a holdown connection device is disclosed in U.S. Pat. No. 5,249,404, the disclosure of which is hereby incorporated by reference. The holdown was originally designed to connect a vertical structural member (e.g. an end post for a shear wall) to a horizontal structural member (e.g. a floor or foundation) thus providing holdown restraint for the vertical member.
Although holdowns may be incorporated into a continuity tie, such connection devices are subject to various inadequacies. These inadequacies are as follows:
Holdown connection devices are generally designed for overall load capacity without regard to overall device deflection. Such connection devices are subject to substantial deflections when loaded. Recent studies of earthquake damaged buildings have recommended that the device deflection of connection devices incorporated in continuity ties be limited in order to allow the continuity tie forces to effectively be transferred through the roof and/or floor diaphragms, and to keep the timber roof and/or floor framing elements from slipping out of their end support devices (joist or purlin hangers). Some building departments have taken these recommendations and have established their own device deflection criteria. One such criterion put forth by the City of Los Angles has reduced the allowable capacity of a holdown device by a factor generally between three and five. Such criteria increases the size, number, and cost of installing continuity ties with holdown connection devices.
Additionally, the design of a holdown connection device is such that all the bolt holes that attach the connection device to the wood structural member are in line thus bearing on the same "grain line" of the wood member. Not only does this accentuate the load on the wood structural member, but if this particular grain line happens to have an abnormality that cannot fully resist the load from the holdown connection device, this particular connection may be compromised and prone to failure.
Another deficiency of the holdown connection device is that the bolt holes bored through the wood member are prone to being oversized, especially in paired applications. Connection devices installed with oversized bolt holes must undergo an additional amount of deflection before the "slop" between the bolt and the wood is taken up. Such deflections affect the structural performance of a building in a manner similar to the deflections associated with a holdown connection device. Recent studies of earthquake damaged buildings have recommended that connection devices to be used in paired applications for continuity ties be installed with drill jigs in order to mitigate the potential for overdrilled bolt holes.
A holdown connection device is generally designed to resist only tension forces between two connected members, and not compression forces. Continuity ties incorporating holdown connection devices typically require that the end bearings between the structural members to be connected and the interceding member be shimmed tight in order to provide compression load capacity. Such shimming is an additional expense to the installation of a continuity tie.
Although there are other deficiencies in the holdown hardware, especially as it compares to the subject invention, these are perhaps the most significant.